How much do i oc

Impossible to say. Assuming the temperatures are acceptable and you're not getting any artifacts, go nuts.

Download FurMark, or any other GPU Stress testing software, and run it until you're confident the OC is stable. If any artifacts appear, go back down to the last stable setting.

Good luck  

When overclocking (Nowadays), It's actually kind of hard to damage cards, as you need them to be running beyond safe temperatures over a fair amount of time. The real issue comes up when dealing with stability, as the higher your clocks are, the more risk there is of your computer crashing.

Some people increase voltage to allow for higher clocks, and use temperature monitoring programs, and stress test programs to monitor the temperature of the gpu under prolonged heavy usage. You can find these programs usually from your manufacturers websites (I.E. EVGA uses the Precision tuner, Eleet, and OC scanner, MSI uses afterburner, etc). I wouldn't know the "considered safety limit" is for your card, but I know that people try not to push nvidia cards beyond 90-100 degrees.

When running tests be sure to monitor temperatures and artifacts. When you get a stable result (no crashing) with no or minimal artifacts, you have yourself a good OC setup. Keep in mind that the harder you push your card, the less lifetime it will have.